Collection of solar energy for use in heating fluids, such as water, is a well known concept with rudimentary systems originating in ancient times. Modern solar heating systems typically incorporate a solar collector that converts the sun's energy to thermal energy and utilize a variety of means to transfer the collected thermal energy into the fluid to be heated, such as for residential, commercial or industrial heating applications.
Solar water heaters may be combined systems or distributed systems. In the case of a combined system, a domestic water storage tank is typically mounted directly to the solar collector. Combined systems are generally not practical in colder climates as the hot water storage tank is cooled by the cold ambient air. In the case of a distributed system, the solar collector is typically located remote from the heated water storage tank, the storage tank being placed in a sheltered location to avoid heat loss to the atmosphere. Distributed solar water heaters are common.
“Direct” solar water heater systems circulate the domestic water to be heated through the solar collector. Direct systems are typically prone to scaling of the collector as a result of the domestic water passing therethrough. Further, direct systems require the collector to be drained when ambient temperatures fall below the freezing point of water (0° C.). Direct systems can be configured as either combined systems or distributed systems.
More sophisticated distributed systems known as “indirect” heating systems circulate a heat transfer fluid or working fluid between the solar collector and a potable water heat exchanger which transfers the solar heat from the working fluid into the potable water. The heat exchanger, such as a tubular coil, may be placed inside a potable water tank for transferring heat from the working fluid circulating through the coil to the water in the tank. Alternatively, the heat exchanger can be located external to the potable water tank, the potable water circulating on one side of the heat exchanger and the working fluid on the other side. Indirect systems typically use a working fluid that comprises agents to reduce scaling and an anti-freeze agent to avoid freezing of the working fluid.
Solar energy can only be harnessed when the sun is shining and some of the heat gained during the day is lost if the potable water or working fluid continues to circulate during nights or during periods of low solar potential. Consequently conventional solar systems require a means for stopping circulation of the working fluid during non-heating conditions. Some systems use a “drain-back” approach that drains the working fluid into a holding tank during the non-heating periods. Systems that don't “drain-back” require enough anti-freeze agent to ensure the working fluid does not freeze up and damage the piping or solar collector.
A significant issue with solar water heating systems is how to mitigate excessive heat. During periods when solar heating of the potable water exceeds the demand for heated potable water, heat will build up in the system. If means for releasing pressure are not provided, excess heat leads to boiling of the working fluid and the resultant pressure increases will rupture the piping or solar collector. Conventionally, overheating is addressed using a number of different mechanisms. “Heat dumps” dissipate excess heat to the atmosphere or through a ground loop or other location. Alternatively, the system is drained back and shut down or the system controller can be manually set to a “vacation” setting that diverts the heat from the potable water system.
Often systems are deliberately under-sized to avoid the overheating challenge. In this case, the solar collector system is sized such that its peak output will provide 90% of the minimum anticipated heat load. As the output of the solar collectors is seasonally dependent, this approach usually results in the solar water heating system contributing about half of the water heating requirement, the remainder being provided through conventional non-solar water heating systems and requiring a reliance on the electric utility grid or other external energy provider. Thus, it is clear in these cases that solar collection is not maximized.
Canadian Patent 1,080,566 to Cummings teaches a solar water heater incorporating a heat rejecting loop to attempt to cool the system. The system is complex and incorporates two separate fluid circuits; one comprising a heat absorbing loop fluidly connected to a heat rejecting loop and the second comprising a heat pickup loop thermally coupled to the solar panel to carry thermal energy away from the panel to the point of use. Circulation of fluid through at least the heat absorbing and heat rejecting loops is solely by gravity and thermal convective effects.
EP 04727915 to Torrens teaches a complex solar collection system in series with a hot water system. A heat dissipater circuit, which may comprise at least part of the panel framework, is used for cooling at least a portion of hot water exiting the solar panels when the water is overheated. The inlet to the heat dissipater is downstream from the solar panels and thus all of the fluid must first be heated and then at least a portion cooled for cooling the system. Torrens relies upon thermosiphon effects in the event of pump failure to ensure all of the water in the system is directed through the heat dissipater to prevent overheating. Applicant believes it is likely that there will be insufficient impetus for thermosiphon within the complex piping of Torrens, resulting in the possibility of overheating of the fluids therein despite the heat dissipation circuit. The Torrens system is particularly unsuitable for use where ambient temperatures fall below freezing as it is a direct system.
Apricus Solar Co. Ltd. (www.apricus.com/html/solar_heat_dissipator.htm) teaches a solar hot water system comprising a fin and tube heat dissipater connected downstream from solar collectors. The system as described utilizes an electrically powered controller and a solenoid valve operated by the controller, to direct overheated fluid from the solar collectors to the heat dissipater. Alternatively, it is mentioned that a thermostatic valve may be used. All of the fluid in the heat transfer circuit is first heated in the solar collector after which at least a portion of the fluid is directed to the heat dissipater for cooling after which the cooled fluid is mixed into the stream of overheated fluid. In cases of peak insolation, sufficient heat may not be released by the heat dissipater. Following heat dissipation, the temperature of the re-mixed working fluid may be inconsistent as the efficiency of the heat dissipater varies with atmospheric conditions. If excessive heat dissipation occurs the efficiency of the system is reduced. If insufficient heat dissipation occurs there remains a risk that the system will over-heat.
Current indirect-distributed systems typically utilize electronic control systems to activate pumps and valves to operate the system. The electronic controller utilizes preprogrammed logic to operate the valves and pumps as conditions determine when to circulate fluid to the solar collector, when to drain-back or load the working fluid, if applicable, when to circulate through an external heat exchanger and when to activate systems which handle excess heat, if available. The operating conditions are measured by electronic temperature and pressure sensors which are connected electrically to the electronic controller. Thus, these control and operating systems require electrical energy which is usually supplied from the electric utility grid. Loss of electrical energy will, at a minimum, cause loss of solar heating. It can also potentially cause damage to the system should the system overheat, result in injuries such as scalding and result in collateral damage to the building such as stained walls and floors caused by overflow of working fluid from ruptured lines and the like.
In order to deal with these problems, some systems provide a battery backup to enable the system and controller to operate for a period of time when the power goes out. In some cases, solar photovoltaic (PV) systems are available to supply the necessary electrical energy either directly to the solar heating system and controller or indirectly, such as through a battery pack.
In addition to requiring electrical energy to operate the solar heating system, electronic control methods are prone to component failure especially when considered in the context of the twenty-year life of a typical solar water heating system. Failure of the electronic control system can lead to piping or component damage and collateral damage similar to that which occurs with the loss of electrical energy. Battery systems also have a shorter life expectancy, usually in the five to ten year range. Failure to test and replace the battery system can lead to same type of damage seen with loss of electrical energy.
Ideally, what is required is a solar water heater system that is simple, efficient and requires no reliance on the electric utility grid or other external energy provider. The solar water heater system should be capable of meeting maximum demand during periods of low insolation without concern of overheating and the resulting potential damage to the systems and structures during periods of high insolation, and particularly during periods where there is also a low demand.